A BELOVED hippopotamus called Maikel died an agonising death in a zoo in Germany after a visitor threw a tennis ball into his enclosure that he mistook for an apple.

Friendly Maikel the Hippo walking through its compound in the zoo in Frankfurt [EPA]

Maikel, 39, wolfed the ball down only to die four days later with his bowel blocked up. Zoo staff said he was in acute agony before his death and had no clue as to what he was suffering from.

He was voted the favourite animal in the zoo by visitors last year and delighted children especially with his gentle nature and graceful swimming.

He died a very painful death. It was an immensely stupid and immensely cruel thing to do, feed a tennis ball to an animal like this

Dr. Anne Nesseler, pathologist

"He was in the best of health and we were frankly baffled," said Professor Manfred Niekisch, director of Frankfurt Zoo. "Our vets did their best for him but it was hard to understand what the 2,000kg animal was suffering from.

"His immense bulk meant an ultrasound was out of the question.

There was no way of us to find the tennis ball in him, let alone remove it."

On Thursday morning he collapsed and died in the water in his enclosure, his lifelong partner Petra swimming repeatedly over his body and trying to nudge him to get up. Because the zoo was concerned for the health of the other hippos a full autopsy was ordered on Maikel at the Hessian State Pathology Laboratory - usually reserved for human victims of crime.

Pathologist Dr. Anne Nesseler led the autopsy on Friday and the tennis ball blocking his bowel was discovered.

"He died a very painful death,"she said. "It was an immensely stupid and immensely cruel thing to do, feed a tennis ball to an animal like this."

A special animal crime squad has been charged by Frankfurt police to find and prosecute his killer under animal protection laws. Virgine Wegner, a spokeswoman for the force, said: "The enquiry is continuing."